The Seventh Element — Wendy Mass

Earth is in danger! The only thing that can save our planet are six essential elements that can fuse into a new source of clean energy. But the elements are scattered throughout the galaxy. And it's up to the Voyagers -- a team of four remarkable kids and an alien -- to gather them all and return to Earth.
 
The Voyagers have made it to the last planet. If they complete this mission, they can finally go home. But they've been in space a long time, and it's starting to take its toll. When one of the crew falls deathly ill, the race back to Earth becomes even more urgent. They just have to combat fire-breathing dragons and an evil alien clone out to sabotage their every move. No big deal.
 
At least the Voyagers have almost every element they need. Only one more to go -- six elements to make the Source and save the world. So why is there a space in the Element Fuser for a seventh?